Among the thirty or so enzymes of detoxication is the group of esterases which are ubiquitously distributed among cells, with about 95 per cent of total activity present in the microsomal fraction of liver. The esterases fall into two classes based on catalytic mechanism. The major activity by far is with the serine-type esterases whereas a minor fraction has a different, as yet unknown, hydrolytic mechanism. An investigation of those enzyme that are in rat liver cytosol has resulted in the isolation of two electrophoretically homogeneous proteins that have a high affinity for aspirin and probably account for most of the hydrolysis of that ester in liver. These are serine-esterases which also catalyze the hydrolysis of a large variety of other esters. A systematic examination of the second group, non-serine esterases, in the cytosol allowed the isolation of twelve esterases of varying molecular size and isoelectric point, all relatively uninhibited by paraoxon, but able to accommodate a very wide range of substrates with an overlapping specificity. Four of these enzymes were prepared in homogenous form and antibody was obtained against three of them; each antibody reacted only with the esterase that served as antigen and not with any of the other eleven esterases. In addition to the oxygen esters, thiol esters served as substrates but amides and peptides did not.